I. Field of the Invention
This invention relates to treating photoconductive compositions, and more particularly, to a method for improving the humidity resistance of photoconductive cadmium sulfoselenide (CdSSe).
II Description of the Prior Art
The forming and developing of images on the surface of certain photoconductive materials by electrostatic means is now well known. Carlson, in U.S. Pat. No. 2,297,691 teaches the basic xerographic process which involves uniformly charging a photoconductive insulating layer and then exposing the layer to a light-and-shadow image which dissipates the charge on the portions of the layer which are exposed to light. The electrostatic latent image formed on the layer corresponds to the configuration of the light-and-shadow image. In another modification, a latent electrostatic image is formed on the photoconductive insulating layer by charging the layer in image configuration. A finely divided developing material comprising a colorant called a toner and a toner carrier is deposited on the image layer. The developing material is normally attracted to those portions of the layer which retain a charge, thereby forming a powder image corresponding to the latent electrostatic image. The powder image may then be transferred to paper or any other receiving surface. The powder image is permanently bonded to the paper by any suitable fixing means. Typically, a heating process called fusing is used. For example, see the fusing processes described in U.S. Pat. Nos. 2,357,809, 2,891,011 and 3,079,342.
It is possible to employ a wide variety of photoconductive insulating materials in the electrostatic process. For example, Carlson, in U.S. Pat. No. 2,297,691 discloses photoconductive insulating materials such as anthracene, sulfur, selenium or mixtures thereof.
These photoconductive materials generally have sensitivity in the blue or near ultraviolet range, and all but selenium have a further limitation of being only slightly light sensitive. For this reason, selenium has been the most commercially accepted material for use in electrophotographic plates. Vitreous selenium, however, while desirable in most aspects, suffers from serious limitations in that its spectral response is somewhat limited to the ultraviolet, blue and green region of the spectrum, and the preparation of vitreous selenium plates requires costly and complex procedures, such as vacuum evaporation. Also, selenium plates require the use of a separate conductive substrate layer, preferably with an additional barrier layer deposited thereon before disposition of the selenium photoconductor. Because of these economic and commercial considerations, there have been many recent efforts towards developing photoconductive insulating materials other than selenium for use in electrophotographic plates.
It has been proposed that various two-component materials be used in photoconductive insulating layers used in electrophotographic plates. For example, the use of inorganic photoconductive pigment dispersed in suitable binder materials to form photoconductive insulating layers is known. It has further been demonstrated that organic photoconductive insulating dyes and a wide variety of polycyclic compounds may be used together with suitable resin materials to form photoconductive insulating layers useful in binder-type plates. In each of these two systems, it is necessary that at least one original component that is used to prepare the photoconductive insulating layer be, itself, a photoconductive insulating material.
There are a number of other photoconductive insulating materials in the prior art which have been used with varying degrees of success including pigments such as cadmium sulfide, cadmium selenide and cadmium sulfoselenide an disclosed in U.S. Pat. Nos. 3,121,006, 3,121,007, 3,151,982, 3,288,604 and 3,109,753. Cadmium sulfoselenide has also been used in conjunction with activator metals such as silver or copper and with or without co-activators, generally halides such as chlorine, bromine and iodine and/or combined with metal impurities or metal salts of zinc as disclosed in British Pat. No. 1,201,128, and with ZnO. The use of a mixed pigment photoconductor (ZnO mixed with CdSSe) can provide an increased light sensitivity over that exhibited by conventional ZnO photoconductors. Still further, compositions of various sulfides and selenides of cadmium and zinc have been used as photoconductors as disclosed, for example, in U.S. Pat. No. 3,121,006.
As an integral part of an electrographic process which employs any of the above referred to photoconductors, the photoconductive surface must exhibit certain electrical properties which, of course, depends upon the characteristics of the electrographic process. One thing that can often adversely affect these electrical properties is humidity. Generally speaking, it is at high levels of relative humidity that the quality of reproduction deteriorates, often to the point that little or no image delination is obtained. Since controlling humidity by external means is generally very expensive, it is considered desirable to have a photoconductive composition which itself has high humidity resistance. Some prior art examples where this problem has been considered are disclosed in U.S. Pat. Nos. 3,736,134, 3,850,632 and 3,912,511.
The ZnO-CdSSe mixed pigment photoconductors that have been formulated from different grades of cadmium sulfoselenide (CdSSe) exhibit varying degrees of humidity sensitivities. As a result of this, the reliability of the electrical properties of the ZnO-CdSSe photoconductors has suffered severely.